Resolution of the age structure of the detrital zircon populations of two Lower Cretaceous sandstones from the Weald of England by fission track dating

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source: https://doi.org/10.7892/boris.115365 | downloaded: 1.2.2022

Resolution of the age structure of the detrital zircon populations of two Lower Cretaceous sandstones from the

Weald of England by fission track dating

A. J. H U R F O R D * , F. J. F I T C H f & A. C L A R K E f

•Laboratory for Isotope Geology, University of Bern, Erlachstrasse 9a, 3012 Bern, Switzerland tDepartment of Geology, Birkbeck College, University of London, 7/15 Gresse Street, London W1P 1PA

(Received!! July 1983; accepted24 October 1983)

Abstract - Modes in the frequency of distribution of fission track ages obtained from detrital zircon grains may prove characteristic of individual sandstone bodies, supporting the identification of the sources from which a particular flow of sedimentary detritus was derived and thus allowing new inferences to be made concerning palaeogeography. A computer program has been written and used to identify modes in the zircon fission track age distribution within two Lower Cretaceous sandstone samples from the Weald of southern England. Pronounced modes appear in one rock around 119 Ma, 160 Ma, 243 Ma and 309 Ma and in the other around 141 Ma, 175 Ma, 257 to 277 Ma and 394 to 453 Ma. The geological implications of these quite dissimilar zircon age spectra are discussed.

It is concluded that they support the palaeogeographical models of Allen (1981) and indicate that the provenance of the first sample, from the Top Ashdown Sandstone member at Dallington in East Sussex, was almost entirely southerly, while that of the second, from the Netherside Sand member at Northchapel in West Sussex, was more varied, but predominantly westerly and northerly.

1. Introduction

The external detector fission track dating method can be used to date individual zircon crystals extracted from a sediment. Statistical evaluation of the individual zircon ages obtained from a detrital population may suggest connection with possible source areas and, thus, assist in palaeogeographic reconstruction. In this study the approach has been applied to zircons separated from samples of two Lower Cretaceous (Wealden) sandstones from Sussex in southern England, kindly made available by P. Allen of Reading University. Analysis of the measured zircon ages, with their estimated statistical errors, makes it possible to determine the probability of occurrence of crystals possessing ages within given intervals.

In his latest provincial model for the Wealden of southern England, Allen (1981) envisages a broad, shallow sedimentary basin bordered by tectonically active, block-faulted, sourcemassifs. Internal tectonism of the same kind may have divided the basin into sub-basins at times. Upfaulting of the bordering massifs generated sandy outwash plains; downfaulting, with concomitant lessening of the relief contrast, led to the development of muddy lake-lagoon-bay environments. The clastic detritus was very largely derived from the erosion of outcrops of old sediments (with some volcanic horizons) underlying thick soils or directly exposed in deep river valleys within the massifs. The principal source areas of the Wealden sediments are thought to have been (i) Londinia to the north, exposing mostly Lower Palaeozoic, Old Red14XS1 I l i a ^ / ^ L / V / k l l l l i k I 1 1 U Ubl I A^\/ IT V I A U l U V U b U I V I ^ ^ A^»

Geol. Mag. 121 (4), 1984, pp. 269-277. Primed in Great Britaii 18

Sandstone, Lower Carboniferous and Upper Jurassic rocks, (ii) Armorica to the south, exposing Precambrian as well as extensive outcrops of Permo-Triassic and Jurassic rocks and (iii) Cornubia to the west, exposing a sequence of Carboniferous, Permo-Triassic and Upper Jurassic rocks. Further to the northwest, both Lower and Upper Palaeozoic rocks were exposed in Hibernia. At times, incursions of the Boreal Sea along the northwestern margin of Londinia may have brought into the Weald sediment ultimately derived from northern Britain and the northern North Sea area. Contemporaneous Lower Cretaceous volcanism is known to have occurred in the Channel Approaches and in the North Sea (Dixon, Fitton & Frost, 1981) and there may have been other nearby centres.

2. Experimental procedure

Zircons were separated from two crushed samples of Wealden sandstone using conventional panning, heavy liquid and magnetic techniques. Sample BK. 1762 (5 kg) came from the Top Ashdown Sandstone member of the Ashdown Beds Formation, outcropping in a shaw east of Hoad's Wood, 1.2 km southwest of the parish church of Dallington in East Sussex (TQ 648185). The rock is lower Wealden, presumably early Valanginian, in age. The analysed material is part of a bulk sample (AWJ 54, locality 14 of Allen, 1947) collected and described by Allen (1949, 1959,1981). A ferruginous sandstone of medium grade carrying reworked glauconite, it does not differ significantly from the description given by Allen in

1949.

Sample BK 1761 (2 kg) came from the Netherside

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Sand member of the Weald Clay Group 0.36 km east of Upper Diddesfold Farm, Northchapel, West Sussex (SU 945298). This horizon is upper Wealden, presumably Hauterivian or Barremian in age. The sampled rock is, again, a ferruginous sandstone of medium grade originally collected by Allen. Detrital tourmaline from this sandstone has been dated previously by the argon-40/argon-39 technique (see below and Allen,

1975, p. 430, sample no. S 8052).

The separated zircon crystals were mounted in FEP Teflon, polished and etched in KOH/NaOH eutectic melt at 230 °C for sufficient time to fully reveal the spontaneous fission tracks, usually between five and ten hours (Gleadow, Hurford & Quaife, 1976). The zircon mounts were then irradiated, each in close contact with a muscovite detector, in the thermal neutron facility Jl of the Herald reactor at Alder- maston, UK, following the procedures of Hurford &

Gleadow (1977). The neutron fluence was monitored by cobalt activation wires, by the inclusion of the NBS dosimeter glass SRM 612, and on the first occasion by including a zircon age standard. After irradiation the external mica detectors were etched in 48% HF at 20 °C for 30 min, to fully reveal the neutron-induced tracks.

For a given crystal, tracks were counted over identical areas of the crystal and its mica detector, using a magnification of 1563 times, under oil immersion. Tracks were counted using a calibrated eyepiece graticule, selecting only zircons with well- etched faces parallel to the c crystallographic axis and possessing low bulk etch rates, the criteria being the visual appearance of the track and the presence of sharply etched polishing scratches with widths less than 1.5 /im. Gleadow & Lovering (1977) have demonstrated that for such crystals a geometry factor of 0.5 is valid for the ratio of track density on an external detector (2n) to the track density on an internal surface (477). The spontaneous-to-induced track count ratio (NJN^) was then available for each crystal, allowing its age to be estimated using the zeta calibration approach (Hurford & Green, 1981).

Hurford & Green (1983) describe the repeated evaluation over seven years of a £ factor for dosimeter glass SRM 612 using four zircons of known age. A grand weighted mean f₆₁₂ value of 339+ 10 (2a-) was derived and has been used in this study.

A probability distribution of the real age about an estimated age requires the use of a notional standard deviation (the 'conventional error' of Green, 1981) which is estimated from the radioactivity counts:

against age for each zircon, using an expression of the

form 1

where Ns, N{ and Na are respectively the spontaneous, induced and SRM 612 detector counts and T is the estimated age. A computer was programmed to establish a normal distribution of probability density

C =

where <r is the notional s.d., calculated as above. The individual probability distributions were summed to form a weighted histogram for the whole zircon population, showing the probable frequency of occurrence of crystals within given age intervals. The form of such histograms is illustrated in Figures la, etc., to be discussed below.

3. Results

Ninety zircons were counted from sample BK 1762 and 44 from BK 1761, with the results set out in Tables 1 and 2. Figures 1 and 2 show the total histograms obtained from the computer analysis. Figures 1 a and la represent the summation of the individual probability distributions for each zircon over the total number of zircons in the sample, applying the notional s.d. to each as described above. In Figures 16.and 2b the discrimination has been enhanced by halving each of the s.d.s. Figures 2c and Id are repeats of Figure la and 2b but omitting ten crystals (marked ' e ' in Table 2) which were slightly below the quality of etching which we would normally select for counting (as described above). The modes identified in the analyses, with the relative heights of their maxima, are set out in Table 3.

For BK 1762 Figure 1 a indicates a concentration of ages around and below 163 Ma (late mid-Jurassic).

The increased resolution of Figure 1 b suggests a pronounced mode around 160 Ma and lesser modes around 119 Ma (mid Lower Cretaceous), 243 Ma (late Permian/early Triassic) and 309 Ma (late Carbonifer- ous). There are also indications of older material, some perhaps going beyond 700 Ma (see Table 1, e.g. HS 14, HS47, ZJ21,ZL 13).

For BK 1761 Figure 2a indicates ages concentrated around 179 Ma (early mid Jurassic) and 253 Ma (Permian). The greater resolution of Figure 2b suggests that the main mode is a little lower (173 Ma) and the next most pronounced around and above 257 Ma, with another around 141 Ma (late Jurassic/

earliest Cretaceous). There is also evidence of older material, between 393 and 475 Ma (Devonian to Ordovician) and some older than 500 Ma (see Table 2, e.g. AA 10, EE 6).

Figures 1c and 2d show the effect of omitting the ten slightly sub-quality crystals (which had been included in an effort to achieve an adequate sample in a rock with few good-quality zircons). The modes indicated by the increased resolution of Figure 2d are at 141 Ma, 175 Ma, 277 Ma (Lower Permian) and 398-453 Ma. Except that the 257 Ma mode now appears rather higher (277 Ma), the results from the two sizes of sample are substantially in agreement. The

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Table 1. Results of fission track dating of detrital zircon grains extracted from a sample (BK 1762) of the Top Ashdown Sand

Crystal

266 139 348 191 118 282 139 48 231 165 187 131 136 213 217 69 135 199 210 52 125 345 197 423 253 125 119 114 210 319 316 150 202 283 113 262 368 195 230 269 250 247 283 358 260 254 139 306 104 146 278 78 174 231 119 176 127 258 113 191

116

88 188 191 238 83 169 191 437 194 137 156

AT, 65 48 52 57 45 39 55 14 19 31 39 65 14 15 116 37 36 115 49 19 24 75 . .37 57 33 20 36 38 20 26 60 45 47 32 33 57 67 51 20 45 75 39 88 33 82 22 72 58 48 66 122 29 38 48 58 84 81 149 22 66

19

42 41 175 86 30 71 140 114 16 38 38

Irradiation FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD84 FTD76 FTD76 FTD76 FTD76 FTD76 FTD76 FTD76 FTD76 FTD88 FTD88 FTD88 FTD88 FTD88 FTD88 FTD88 FTD88 FTD88 FTD88 FTD88 FTD88 FTD88 FTD111 FTD 111

FTD111

FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 FTD HI FTD 104 FTD 104 FTD 104

Pd/10⁵* (t cm"²) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313 (2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.313(2520) 3.186(2440) 3.186(2440) 3.186(2440) 3.186(2440) 3.186(2440) 3.186(2440) 3.186(2440) 3.186(2440) 4.207(2154) 4.207(2154) 4.207(2154) 4.207(2154) 4.207(2154) 4.207(2154) 4.207(2154) 4.207(2154) 4.207(2154) 4.207(2154) 4.207(2154) 4.207(2154) 4.207(2154) 5.17t (700) 5.17f(700)

5.17f (700)

5.17t(70O) 5.17f (700) 5.17t (700) 5.17f(700) 5.17f(700) 5.171(700) 5.17t (700) 5.17f(7OO) 3.20 (662) 3.20 (662) 3.20 (662)

Age (Ma)

226 161 365 185 146 394 140 190 649 292 264 112 524 752 104 104 207 96 236 152 286 253 292 404 417 342 183 166 564 655 289 184 237 478 189 253 301 203 593 315 178 333 171 561 169 775 136 366 153 156 160 189 318 334 145 148 111 122 435 249

514

181 390 95 238 238 205 118 327 626 193 219

(Ma)s.d.

32 27 55 28 26 68 23 58 155 57 47 17 147 201 12 21 39 11 38 41 64 33 53 58 78 83 35 31 132 134 41 32 39 90 38 37 40 32 139 51 24 58 21 103 22 173 20 53 27 23 18 41 57 54 23 20 16 13 103 37

129

35 69 11 31 51 30 14 37 165 36 40 HS 1

HS2 HS3 HS4 HS5 HS6 HS7 HS8 HS9 HS 10 HSU HS12 HS13 HS 14 HS 15 HS 16 HS 18 HS 19 HS20 HS21 HS22 HS23 HS24 HS25 HS26 HS27 HS28 HS29 HS30 HS31 HS32 HS33 HS34 HS35 HS36 HS37 HS38 HS39 HS40 HS41 HS42 HS43 HS44 HS45 HS46 HS47 HS48 HS49 HS50 HS51 HS52 HS53 HS54 HS56 HS57 HS58 HS59 HS60 ZA3 ZA7 ZA10 ZA 11 ZA 14 ZB4 ZB6 ZB7 ZB 16 ZB 19 ZB29 ZD13 ZE 1 ZE7

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Table 1. (cont.)

Crystal N. N, Irradiation (t cur²)

Age (Ma)

s.d.

(Ma) ZE8

ZE 10 ZE 14 ZI2 ZI 16 ZJ 1 ZJ 13 ZJ21 ZJ26 ZJ27 ZJ44 Z L 4 ZL 10 ZL12 ZL13 ZL 14 ZL16 ZL28

313 103 211 556 106 184 528 195 84 102 85 145 160 168 232 125 84 152

111 34 71 110 53 87 186 20 20 33 66 20 111 30 21 50 22 101

FTD 104 FTD 104 FTD 104 FTD 104 FTD 104 FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 FTD 111

3.20 (662) 3.20 (662) 3.20 (662) 3.20 (662) 3.20 (662) 5.17t(700) 5.17t(70O) 5.17f(700) 5.17f(700) 5.17f(700) 5.17f(700) 5.17f(700) 5.17t(70O) 5.17t(700) 5.17f(700) 5.17f(7O0) 5.17f(700) 5.17f(700)

151 162 159 268 108 183 244 802 358 265 112 605 125 473 902 215 326 131

18 33 23 30 19 25 23 191 90 54 19 146 16 95 203 37 79 17

* pA is the detector track density from the glass standard dosimeter (tracks per cm²). No. of tracks counted shown in brackets.

t Inferred notional value obtained by including zircons HS 23-28 in the irradiation as a standard and comparing with pA for SRM 612 in irradiation FTD 84.

Table 2. Results of fission track dating of detrital zircon grains extracted from a sample (BK 1761) of the Netherside Sand

Crystal Irradiation

Pd/10⁶' (t cm"²)

Age (Ma)

s.d.

(Ma) AA 1

AA2 AA3 AA4 AA5 AA6 AA9 AA 10 AA16 AA 17 AA18 AA20 BB6 BB 16 BB21 BB25 CC1 CC4 CC24 CC25 DD2 DD7 DD8 DD9 DD11 DD12 DD13 EE2 EE5 EE6 FF10 FF20 FF23 GG3 GG4 GG5 GG6 GG9 GG10 GG13 GG16 GG17 GG19 GG20

138 446 169 144 104 179 274 154 404 168 222 114

300 218

266 266

270

116 383 309 228 81 67 94 85 96 214 239 369 178 236 355 92 184 149 130 129 80 58 51 102 130 143 112

41 86 71 25 34 53 85 22 70 31 60 81 67 61 58 133 107 40 169 173 102 40 52 34 21 40 45 155 232 35 192 180 67 69 40 43 66 18 33 26 57 38 50 44

FTD 111 FTD 111 FTD 111 FTD HI FTD HI FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 FTD 111 F T D 111 F T D 111 F T D 111 FTD 111 FTD 111 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 110 FTD 116 FTD 116 FTD 116 FTD 116 FTD 116 FTD 116 FTD 116 FTD 116 FTD 116 FTD 116 FTD 116

5.17f(700) 5.17f (700) 5.17f(700) 5.17t(70O) 5.17t(700) 5.17f(700) 5.17t(700) 5.17f(70O) 5.17f(700) 5.17f(700) 5.17f (700) 5.17f(70O) 5.17t(700) 5.17T(700) 5.17T (700) 5.17t(700) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 6.70(1394) 5.38(1119) 5.38(1119) 5.38(1119) 5.38(1119) 5.38(1119) 5.38(1119) 5.38(1119) 5.38(1119) 5.38(1119) 5.38(1119) 5.38(1119)

288 439 205 486 263 289 276 586 487 458 316 122 381

301

390 173 280 321 252 200 249 226 145 307 444 267 519 173 178 553 138 220 154 239 331 270 176 393 158 176 161 305 256 228

52 54 30 107 53 47 36 135 66 91 48e

18e 53

45

58 19 33 60 24 20 30 44e 27 62e 109

51e 86 18 16 103 14 21 25 34e 60 48 27 103 35 43 27e 57e 43 e 41e ' pd is the detector track density from the standard glass dosimeter (tracks per cm²). No. of tracks counted shown in brackets.

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0 100 200 300 400 500 600 Ma Fig. 1a

Figure la. Frequency distribution BK 1762, 90 crystals.

Peak at 163 Ma.

0 100 200 300 400 500 600 Ma Fig. 1b

Figure I b. As Figure I a but s.d. multiplied by 0.5. Peaks at 119, 160, 243, 309* Ma. *Maximum not well defined.

findings from Figures 1 and 2 are set out in summary form in Table 3.

4. Geochronometric discussion

The computer analysis was aimed at extracting the greatest amount of information available, in the form of a most likely identification of any groupings of ages

of the zircons in the sandstone. It became apparent that the most secure statistical approach was to aim at a large size of sample. This was achieved for BK 1762, but for BK 1761 the sample is limited by the scarcity of zircons of adequate quality. The results for the latter rock must therefore be considered more tentative than those for BK 1762.

The strongest indications of ages of source material are, for BK 1762 around 160 Ma (late mid Jurassic), and for BK 1761 around 175 Ma (early mid Jurassic) and 257-277 Ma (Permian). Both rocks show evidence of older material, and (in the case of BK 1762) some younger material encroaching near to or below the accepted age of early Wealden sedimentation. The oldest ages suggest a wide range of possible origin for some of the zircons. The very youngest ages are more difficult to explain. They may indicate that some zircons were inadequately etched despite the care that was taken. There is no reason to suspect contamination at any stage.

5. Palaeogeographical discussion

In the Weald of southern England, Allen (1975, 1981) suggests that two episodes of major fan-building produced the Ashdown and Lower Tunbridge Wells sandy outwash plains in the Lower Cretaceous. During both episodes, outwash spreading south from Londinia encountered and intermingled with similar deposits building north from Armorica. Allen's interpretation is based upon a careful study of the larger clasts and mineral suites contained in these rocks. For example, he finds that in Lower Wealden times sandy detritus

«

Fig.2a

Figure 2a. Frequency distribution BK 1761, 44 crystals.

Peaks at 179, 253 Ma.

0 100 200 300 400 500 600 Ma Fig.2c

Figure 2c. Frequency distribution BK 1761, 34 crystals.

Peaks at 177, 259 Ma.

I

400

Flg.2b

Figure 2b. As figure 2a but s.d. multiplied by 0.5. Peaks at 141, 173, 257, 393*-475* Ma. *Maximum not well defined.

600 Ma

Fig.2d

Figure 2d. As Figure 2c but s.d. multiplied by 0.5. Peaks at 141, 175, 277, 398*-453* Ma. 'Maximum not well denned.

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Table 3. Frequency modes indicated by the histograms

Rock BK 1762 BK 1762 BK 1761 BK 1761 BK 1761 BK 1761

No. of zircons 90 90 44 44 34 34

s.d.

multiplier 1 0.5

1 0.5

See Fig.:

\a

\b la 2b 2c Id

Age frequency modes (Ma) (relative heights of maxima in ] 119

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—

— 141 (59)

—

— 141 (58)

163 160 (100)

179 (100)

173 (100)

177 (100)

175 (100)

243 (56) 253 (86) 257 (77) 259 (68) 277 (63)

parentheses 309*

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—

— 393*^75*

(22, 25)

—

— 398*-453»

(24, 25) Relative heights of maxima are in terms of largest = 100.

* Maximum not well defined, see Figures.

with high staurolite/kyanite ratios, indicative of high-grade metamorphic source rocks, entered southern England from the south but is confined to Sussex. Other lines of evidence include the petrography of the larger clasts and the argon-40/argon-39 ages of detrital tourmaline and other minerals (Allen, 1949,

1972, 1975, 1981).

A stratigraphical comparison between the age distribution of detrital zircons in the two Wealden sandstone samples we examined is given in Table 4.

Zircon is a common accessory mineral in many volcanic and plutonic igneous rocks and in some metamorphic rocks. It is a resistant mineral during denudation. Thus detrital zircon is also common and may be reworked through many sedimentary cycles.

Numerous reworked zircon grains are likely to be present in our samples. Nevertheless, it is most probable that the principal modal concentrations in the zircon age spectra represent either direct derivation from rock outcrops of the appropriate ages or reworkingthroughintermediatesedimentsoutcropping on approximately the same azimuth as the ultimate source rocks. Thus, taking into consideration the known Upper Palaeozoic and Mesozoic geology of western Europe, it would be reasonable to expect that outcrops of penecontemporaneous Lower Cretaceous,

late mid Jurassic, late Permian/early Triassic and late Carboniferous rocks were important in the derivation of the Top Ashdown Sandstone sample and that, in addition, small areas of older Palaeozoic and Precambrian rocks might have been exposed at this time. Alternatively, some or all of these recognizable zircon age groups could have been concentrated in, and then reworked from, sedimentary rocks of intermediate age and location outcropping in the source massifs during lower Wealden times. The absence of prominent concentrations of zircons of late Jurassic/early Cretaceous, Lower Permian or Cale- donian age is also important in characterizing the provenance of this rock.

The necessary combination of primary and/or secondary source outcrops to provide the zircon age distribution found in the Top Ashdown Sandstone was not present in the Londinia massif in early Wealden times because Permo-Trias, Lower and Middle Juras- sic strata and Variscan granites must be virtually absent from the Londinian succession. To the south and southeast, however, rocks of the required ages are present, either in Armorica or beyond. Mid Jurassic volcanoes have been reported in the Massif Central and extensive late Permian/early Triassic volcanics occur in Aquitaine, Biscay and Iberia (Carte geologique

Table 4. Stratigraphical comparison between the age distribution of detrital zircons in the two samples of Lower Cretaceous sandstones (see Figures 16 and 2d)

BK 1762 Dallington

BK 1761 Netherside mid Lower Cretaceous

late Jurassic/earliest Cretaceous.

mid Upper Jurassic late mid Jurassic early mid Jurassic

late Triassic/earliest Jurassic. . . early Upper Triassic.

late Permian/early T r i a s s i c . . . . Lower Permian

late Carboniferous early Carboniferous Caledonian Precambrian

.peak . . .trough.

.major peak .trough. . . .

.trough, .peak. . .trough.

.peak . . .trough, .peak . .

.major peak .trough. . . . .peak .trough, .peak . . .present .present

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de la France et de la marge continentale, 1980).

Suitable late Carboniferous, older Palaeozoic and Precambrian source rocks outcrop in Armorica itself.

Whilst the younger zircons are almost certainly derived from volcanics and their associated intrusions, the late Carboniferous zircons are more likely to have their main source in the granites and related post-orogenic intrusions of the Armorican sector of the Variscan fold belt (granites, mostly between 300 and 330 Ma, Chauris et al. 1956; Graindor &

Wasserburg, 1962; Leutwein et al. 1969; and lamprophyres, around 300 Ma, e.g. Lees, 1974). The actual source volcanoes from which the Lower Cretaceous zircons were derived are as yet unknown, but explosive eruptions from volcanoes in the Channel Approaches may have spread ash over wide areas (Jeans et al. 1982). (Lower Cretaceous volcanism is also known in the northern North Sea, see Dixon, Fitton& Frost 1981.)

The Bathonian Fuller's Earth of southern Britain is not regarded as a probable source of the large population of mid Jurassic zircons in the Top Ashdown sample because at its present outcrops it is almost free of zircon (R. J. Merriman pers. comm., and our own observations) and was probably poorly exposed in early Wealden times. Major occurrences of mid Jurassic basic volcanism (with ages around 165 Ma) are known in both the Celtic and North Sea areas (Howitt, Aston & Jaque, 1975; Woodhall &

Knox, 1979; Harrison et al. 1979, Dixon, Fitton &

Frost, 1981) and the possibility that zircons of this age were introduced into the Weald from either Londinia or Armorica after reworking from Upper Jurassic sandstones cannot be ignored entirely. If, however, part or all of the late mid Jurassic zircon component of the Top Ashdown Sandstone was derived from either the north or west, it is puzzling that there is no evidence of late Jurassic/early Cretaceous (probably derived from the southern North Sea), Lower Permian (from Cornubia) and broadly Caledonian zircon components, all of which appear in the Netherside Sand sample. Thus, it would appear that examination of the age spectrum of the detrital zircon population of the Top Ashdown Sandstone sample does provide strong confirmatory evidence of Allen's southerly derivation for much, if not all, of its clastic detritus.

In later Wealden times in the western Weald, environmental conditions were such as to make deposition predominantly argillaceous. Short-lived arenaceous incursions did occur, however, and the Netherside Sand is one of several such intercalations within the Weald Clay Group. Allen (1981) demonstrated the appearance of a major detrital component derived from Cornubia in these sands, and a less important component that appears to have been brought in from the north along the northwestern shoreline of Londinia by invasions of the Boreal Sea.

Armorican debris seems totally absent. Argon-

40/argon-39 ages of 258± 11, 207± 16 or 237+ 11 and

> 372 + 40 Ma obtained from detrital tourmaline (Allen, 1975) indicate a combination of Permo-Triassic and late Caledonian source rocks.

The zircon age spectrum of our Netherside Sand sample is quite different from that of the Top Ashdown Sandstone: it suggests either that extensive outcrops of late Jurassic/early Cretaceous, early mid Jurassic and early Permian volcanics are likely to have been present in its source areas or that zircons derived from them have been reworked from intermediate sediments. Zircons derived from the Caledonian fold belt are an important minor component. Precambrian zircons derived either directly or by reworking are also present as a minor component. Whilst early Permian volcanism is known in Cornubia (Exeter volcanics) (Tidmarsh, 1932; Miller, Shibata & Munro, 1962;

Miller & Mohr, 1964) and volcanic clasts in the Lower Permian breccias and conglomerates of Devon (see Hatch, Wells & Wells, 1961, p. 484; Laming, 1966;

Cosgrove & Elliott, 1976), neither mid nor late Jurassic volcanism has been recorded there. The source vents from which the Bathonian Fuller's Earth was erupted are unknown. The mid Jurassic zircons in our sample could have the same derivation. Nevertheless, it is still possible that many of the Jurassic and older zircons in the Netherside Sand were derived ultimately from further afield, either from the northern North Sea and from Caledonian Britain via the agency of the arm of the Boreal Sea, or from volcanism occurring in the Fastnet/Channel Approaches area or from as far away as the Mid Atlantic Rift. Possible source volcanics of Lower Permian and Mid Jurassic ages are widespread in the North Sea (Dixon, Fitton & Frost, 1981), but we have seen no reports of volcanism of late Jurassic/earliest Cretaceous age from the northern North Sea. Jurassic volcanism occurred frequently along the Mid Atlantic Rift zone. The Zuidwal volcano in Holland is 144 Ma old (Dixon, Fitton &

Frost, 1981) and ashes of this age could have been reworked from Londinia and elsewhere in the north to provide a zircon component around that age consistent with our 141 Ma peak. Thus, it would appear that the zircon age spectrum of the Netherside Sand probably indicates a combination of Cornubian, Londinian and more distant westerly and/or northerly provenance. The indications of southerly provenance, so clearly seen in the Top Ashdown Sandstone, are totally absent. The palaeogeographical conclusions outlined above are compatible with the model proposed by Allen (1981).

6. Conclusions

Fission track ages were obtained from the individual detrital zircon crystals in the heavy mineral concentrates extracted from two Lower Cretaceous sandstones occurring in the Weald of England. The distribution

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of ages within the two populations was analysed taking into account the estimated uncertainty of each individual age determination. The Top Ashdown Sandstone sample (BK 1762) shows a marked concentration of zircon ages around 160 Ma (late Middle Jurassic) and lesser concentrations at both younger (119 Ma) and older ages (243 Ma and 309 Ma). These zircon concentrations can be compared with the groupings of ages in figure 10 of Allen, 1981, around 155 Ma, 230 Ma and 315 Ma. The contrast in relative abundance between Variscan and mid Jurassic ages in the two samples presumably results from the virtual absence of tourmaline in the mid Jurassic volcanic source rocks. Much older, often metamict, zircons are also present. The oldest datable crystal had an apparent age of 902 + 203 Ma. This compares well with the two oldest (tourmaline) dates in figure 10 of Allen, 1981, one of which, from the top Lower Tunbridge Wells Pebble Bed, was 918 + 83 Ma (Allen, 1975, p. 430). The distribution of zircon fission track ages from the Netherside Sand sample (BK 1761) on the other hand shows (slightly less certain) peak concentrations around 175 Ma (early mid Jurassic) and 257-277 Ma (Lower Permian) with lesser concentrations around 141 Ma (late Jurassic/earliest Cretac- eous) and between 398 and 453 Ma (Caledonian).

Older and metamict zircons are present also in this rock.

As would be expected, the results obtained above show that the detrital zircons in these two Lower Cretaceous sandstones range from Mesozoic to Precambrian age. Nevertheless, it seems clear that, at the localities sampled, each horizon shows a quite different and distinct spectrum of zircon ages.

Although much further work remains to be done, the results of this pilot study do prompt the suggestion that fission-track age spectra obtained from one or more of their detrital mineral populations may enable the characterization of individual sediments in a way that will prove useful for correlation purposes. The spectrum obtained from any rock that has not undergone post-depositional heating (due to deep burial or metamorphism) of sufficient intensity and duration to cause the annealing of fission tracks in its constituent mineral must be related directly to the provenance of the clastic detritus. The Wealden of southern England meets these requirements. It appears never to have been buried to more than 2 km:

its cover probably never exceeded 1.5 km (Allen, 1981).

Thus, the zircon age spectra of the two rocks sampled can be used to show that (i) the source of the clastic detritus in the Top Ashdown Sandstone sample from Dallington was very largely to the south and southwest, in Armorica and beyond, and (ii) the provenance of the Netherside Sand sample from Northchapel is to be found in a mixture of westerly (Cornubian or even further afield) and northerly

(northern Britain, the North Sea and/or Londinian) sources, thus supporting Allen's suggestions in 1975 and 1981.

Acknowledgments. The work was carried out at Birkbeck College, University of London, and partially supported by the N.E.R.C. A.J.H. acknowledges current financial support for fission track dating in Bern from the National- fonds zur Forderung der wissenschaftlichen Forschung. The sandstone samples were provided, as described, by P. Allen who also commented on a draft of this paper. Irradiations were carried out in the Herald reactor at Aldermaston by C. George and M. Hynes, and paid for by the N.E.R.C.

Advice on computation was given by P. Hooker and J. R. Wheldon. A. Carter assisted with the computation and other work at Birkbeck College. The work was programmed for the CDC 6000/6600 computer situated at the University of London Computer Centre and use was also made of the Sinclair ZX 81 and ZX Spectrum computers.

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